Flyback converter has been widely used in low power supplies, especially in power supply below 100 W, due to its simple structure and low cost, etc.
For the flyback converter with quasi-resonant (QR) control mode and discontinuous current mode (DCM), the relation between the output power Po and the switching frequency fs satisfies the following formula:Po=½LmIpk2fs  (1)Wherein Po is the output power; Lm is the inductance value of primary winding of a transformer; Ipk is a peak value of the current flowing through a primary side switch; fs is the switching frequency of the primary side switch. For a given output power Po, the switching frequency fs may be reduced by setting a larger peak value of the current Ipk.
Further, a VCO (Voltage Controlled Oscillator) frequency reduction control mode is provided in related art. A lower switching frequency fs at light-load condition may be obtained by setting a larger preset current value IPK_VCO, according to which the peak value of the current Ipk is controlled. The Formula (2) is obtained as below:
                              f          s                =                              2                                          L                m                            ⁢                              I                                  PK                  ⁢                                                                          ⁢                  _                  ⁢                                                                          ⁢                  VCO                                2                                              ⁢                      P            o                                              (        2        )            
For a fixed preset current value IPK_VCO, the switching frequency fs is proportional to the output power Po. When the output power Po is reduced to a certain value, the switching frequency fs may be reduced below 20 kHz and enters into an audio frequency range, which causes acoustic noise. In order to resolve this issue, a control method of burst mode is provided. When fs is reduced to a preset frequency value fs_BST (fs_BT>20 kHz), fs remains unchanged. Then the output power Po can be decreased by reducing the working time of the flyback converter.
In the burst mode, the output power Po satisfies the following formula:Po=EBSTfBST  (3)Wherein EBST represents energy transferred by each cluster of pulse train, fBST represents frequency of the cluster of pulse train, as shown in FIG. 1. Generally, the larger the energy, the louder the acoustic noise. For a certain burst mode frequency fBST from Formula (3), the energy EBST transferred by each cluster of pulse train is proportional to the output power Po.
Therefore, in a non-burst mode, for any output power Po, the larger the preset current value IPK_VCO, the smaller the switching frequency fs, and the higher efficiency at light load condition can be achieved referring to Formula (2). However, when entering into the burst mode, as shown in FIG. 1, the fs is arranged to be at fs_BST and remains unchanged, so the larger the preset current value IPK_VCO, the larger output power Po. For a fixed fBST, the larger the output power Po, the larger the energy EBST transferred by each cluster of pulse train, and the louder the acoustic noise.
So for better efficiency at light load condition, the preset current value IPK_VCO needed to be set higher. However, for better acoustic noise performance, the preset current value IPK_VCO needs to be set lower. Therefore, there is a dilemma between efficiency and acoustic noise with conventional control methods.